1. Field of the Invention
The invention relates to a firing circuit for a motor vehicle occupant protection system.
2. Description of the Related Art
Such a firing circuit is already in practical use and serves for triggering the protection system in the event of a motor vehicle accident (which is about to happen or has just occurred). The motor vehicle occupant protection system may in this case comprise one or more airbags, for example front airbags and side airbags, a belt-tensioning system and/or other components protecting the occupants of the motor vehicle in an accident.
To avoid inappropriate triggering of the occupant protection system, in addition to the crash sensors, for example acceleration sensors, the firing circuit is usually provided with a safety sensor, which serves as a redundant crash sensor and, for example, closes, and consequently allows the activation of the primer, only after a predetermined minimum acceleration value is exceeded. The safety sensor lies in series with the firing stage activating the primer, so that the primer current flows via the safety sensor. The safety sensor and its switching contacts must consequently be able to withstand high currents in order that they can carry the firing current essentially without any loss or interruption. Moreover, the safety sensor must be designed in such a way that it maintains its switched-on state continuously for at least a certain minimum time period which must be equal to or greater than the minimum current-flow time period necessary for reliable primer firing.
In order that a safety function is also provided for the second and possibly further firing stages in occupant protection components which can be triggered in multiple stages, each firing stage may be equipped with its own mechanical safety switch, which however gives rise to high expenditure. If, on the other hand, only a single safety switch is used for all the firing stages of this occupant protection component, it is not possible for the safety function to be designed optimally for the individual firing stages, for example the respective safety time windows during which the triggering of the individual firing stages is permitted.
The invention is based on the object of providing for a motor vehicle occupant protection system with at least one occupant protection component which can be fired in multiple stages an electric firing circuit which makes good adaptation of the safety functions possible for the firing stages of the occupant protection component which can be triggered in multiple stages.
This object is achieved by the features mentioned in patent claim 1.
Advantageous configurations of the invention are specified in the subclaims.
In the invention there is an electronic switching element which is controlled by the safety switch and is connected in series with the firing stage, that is to say carries the firing current when the primer is triggered. The electronic switching element has no mechanical internal switching contacts and can be designed without any problem to withstand the appropriate current load, so that essentially loss-free, reliable carrying of the firing current is ensured. This configuration at the same time facilitates the integration of the switching element into a firing circuit IC, which contains for example the electronic control device and the firing transistor, controlled by the latter and connected in series with the primer. The safety switch now only has to carry the driver signal, but no longer the firing current, and, with a significantly reduced nominal current intensity, can consequently be made in a very compact and low-loss type of design.
Preferably connected between the safety switch and the switching element is a timing element, in the simplest case a holding circuit, which maintains the switching element driver signal for a certain time period, corresponding at least to the firing-current flow period necessary for reliable primer firing. This ensures that the switching element remains reliably switched-through for the necessary minimum time period, even if the safety switch happens to have already opened again prematurely. As a result, the requirements which the safety switch has to meet are at the same time reduced considerably, since no specific minimum closing time period has to be ensured. Even only brief closing of the safety switch is sufficient to allow the firing to be executed via the holding element and the electronic switching element at a desired firing instant, fixed by the control device, and with the necessary current flow period within the time window, which is dictated by the timing element and is made to be of an adequate size.
The timing element may also provide alternatively, but preferably additionally, a time delay function, so that the switching element is activated only with a certain time delay after closing of the safety switch. This may be of advantage in particular in the case of multi-stage firing circuits containing a plurality of firing stages if they are to be fired at different times. The safety switch may in this case be connected directly to a further firing stage and allow activation of the latter during its closed state, while the safety time window for that firing stage which is coupled to the electronic switching element is opened only after a time delay and consequently firing of the primer of this firing stage (controlled by the control device controlling this firing stage) is possible only after the beginning of this time window.
The firing stages are preferably provided with separate, own energy stores, so that in each case only the firing currents of the assigned primers flow via the switching elements providing the safety function (mechanical or electrical safety sensor or electronic switching element). Consequently, the maximum current loading of the individual safety elements is significantly less than if a single safety sensor is provided for all the firing circuits, so that these safety elements can be designed for a lower current carrying capacity. Furthermore, on account of this substantial decoupling, even in the event of a fault in the switching through of the safety element of one stage there is still a certain probability that at least the safety function of the other firing stage is correctly ensured and consequently the primer of the latter can be triggered at the correct time.
In particular in the case of a single-stage firing circuit, the safety switch may be formed by a simple mechanical trigger switch, for example a micromechanical acceleration switch, which merely has to generate a weak switching signal when the predetermined limit value of the physical parameter to be sensed, for example the acceleration, is reached or exceeded and which also does not have to have its own signal-holding capability, for example hysteresis. The safety switch can consequently be designed in a miniaturized form for only a low current carrying capacity, so that the overall dimensions of the firing circuit can be reduced. The safety switch may, however, also be designed as a sensor with a downstream integrator and threshold-value decision circuit, in which an acceleration-dependent output signal of the sensor is also integrated and is compared with a threshold value.
The electronic switching element may be of any desired design, provided that its ability to be controlled by a driver signal and a current carrying capacity adequate for the firing current are ensured. A transistor, for example a field-effect transistor, is preferably used, being available with high functional reliability, low expenditure and preferably also in an integrated design.
In the case of the invention there is for the second firing stage an electronic safety switching element, the switching state of which is controlled in dependence on two different input variables (switching signals). This makes it possible for the safety function of the second firing stage to be specifically fixed. The two switching signals are preferably formed in dependence on the switching state of the mechanical safety switch and with the firing triggering decision of the control device for the second firing stage taken into account. This allows not only increased redundancy with corresponding improvement in immunity to inappropriate triggering to be achieved but also the safety function to be sensitively adapted to the current evolvement of an accident.
The safety switching element may be formed, for example, by two power switching elements which are connected to each other, for example connected in series, the control inputs of which are subjected separately to respectively differently formed control signals and which release the primer firing current only when both switching elements are activated. Alternatively, it is also possible to provide in the primer firing current only a single safety switching element, designed as a circuit breaker, and to fix its control signal in dependence on two control signals which have, for example, been combined by an AND operation.
The one switching signal may be formed by a timing element, in particular a holding circuit, which generates the one switching signal when the mechanical safety switch is closed, but still maintains it for a certain time interval even after opening of the safety switch. The other switching signal may likewise be formed by a timing element, such as a holding circuit for instance, which is controlled by the firing signal generated by the control device for firing the second firing stage and is designed such that it generates the second switching signal for a certain time period even after ending of the firing signal. Consequently, activation of the safety switching element is ensured for an adequate duration even after opening of the mechanical safety switch or after ending of the firing signal for the second stage. If, however, on the other hand, there is an excessive time interval between the reopening of the mechanical safety switch and the firing decision for firing the second firing stage, the two switching signals are not generated with an overlap, so that the safety switching element is not activated and consequently a good safety function is ensured in the sense of suppressing inappropriate triggering.
The holding circuits are preferably designed as a component part of the control device, which itself contains the triggering algorithm for the second firing stage and also the firing routine.
The two switching signals are emitted to separate outputs of the control device, preferably designed in the form of a microcontroller or microprocessor, and are then combined externally with each other to achieve the safety function. This further increases the immunity to inappropriate triggering in comparison with a case in which only a single output for generating a single switching signal would be generated for the switching through of the safety switching element. This is because there is a very low probability that, in the event of malfunctions, the control device will firstly generate at both switching signal outputs such signals that the safety switching element is activated and will secondly also generate at the same time firing signals for the actual firing of the primer.
To increase the probability of firing when firing of the second firing stage is required, the power supply of the latter is preferably of a redundant design, that is on the one hand via the supply link containing the safety switching element and on the other hand via a power supply provided directly by the mechanical safety switch, preferably via a rectifier diode. This latter power supply consequently represents to a certain extent a bypass for the safety switching element, which however is activated only during the closed state of the mechanical safety switch and consequently provides, at least during this critical phase, a redundant power supply for the second firing stage as and when it is required.